dwipreproc: Final fixes to b=0 first volume enforcement
dwipreproc: Final fixes to b=0 first volume enforcement

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3